The present invention relates in general to combustion processes and, in particular, to a new and useful system for controlling combustibles; in particular, carbon monoxide, and oxygen in the flue gases of a combustion process which is supplied with a fuel/air mixture.
A number of methods are known for controlling combustion efficiency by analyzing the composition of flue gases generated by a combusion process.
It is known to control, or trim, the fuel/air ratio using an oxygen control loop or a carbon monoxide control loop.
Control of combustion efficiency using an oxygen control loop has shortcomings in that a zirconium oxide oxygen sensor cannot determine if the fuel and air are burned. The actual fuel/air ratio is being measured and this is a measurement of efficiency only if complete combustion takes place (which is normally not the case).
If carbon monoxide, or other combustible, is sensed for the purpose of controlling the combustion process, the fuel/air ratio is not determined nor is the actual efficiency of the burning. Combustible sensing merely tells the amount of unburned fuel left over from combustion. A control using a carbon monoxide or combustible measurement alone cannot tell whether more air improves combustion or just dilutes the the flue gases to lower the carbon monoxide content of the flue gases. The increased amount of air required, if combustion is improved, is extremely small but a large amount is required to dilute the carbon monoxide back to set point when combustion does not improve with added air. Thus, an increasingly inefficient combustion may be taking place even when reduced amount of carbon monoxide are sensed in the flue gases.
U.S. Pat. No. 4,231,733 to Hickman, et al, shows a method of measuring oxygen and combustibles for use in adjusting a fuel/air ratio control. This method has the same shortcomings as the oxygen control described above. It also has a further shortcoming in that is cannot measure oxygen and combustibles at the same time. The Hickman, et al, patent discloses a scheme for switching from oxygen sensing to combustible sensing at a selected point. In fact, both oxygen and combustibles are almost always found together when a combustion process is operated at its most efficient combustion point. Only in a very few cases, where combustion is carried out under pressures higher than 50 psig is it possible to have extremely low excess oxygen with no combustibles.
U.S. Pat. No. 4,162,889 to Shigemura discloses a method and apparatus for controlling combustion efficiency where oxygen and combustibles are sensed in a flue gas. A flow measurement and a quality of fuel measurement must also be utilized to achieve the control function, however.
U.S. Pat. No. 4,330,260 to Jorgensen, et al, discloses a method and apparatus for regulating combustion in a furnace which utilizes an oxygen sensor as well as an optional carbon dioxide sensor for achieving a conrol function. The speed of a blower is regulated according to this patent to effect optimum combustion efficiency. The sensing of a combustible content, and in particular the amount of carbon monoxide left in the flue gases, is not disclosed.